15) D

Half-life of X = 4.8 hours
Number of half lives of X in 24 hours = 24 ÷ 4.8 = 5
Count rate of X after 5 half lives = 320 ÷ 2⁵ = 10

Half-life of Y = 8 hours
Number of half lives of X in 24 hours = 24 ÷ 8 = 3
Count rate of Y after 3 half lives = 480 ÷ 2³ = 60

Total count rate = 10 + 60 = 70